Field of the Invention
The invention lies in the telecommunications field. More specifically, the invention relates to a method for measuring the characteristics of radio channels, in which the signals are received by a total of M1 receiving sensors in a linear antenna array, wherein the respective received signals are composed of wave elements of a transmitted signal with a different incidence direction and different delay. The invention furthermore relates to a measurement configuration for measuring the characteristics of radio channels having a linear antenna array, having a number of antenna sensors. Each antenna sensor is followed (in a signal flow direction) by analog/digital sampling, a filter matched to the signal, a stage for discrete Fourier transformation, and at least one signal processor is provided for the reception stages.
In a large number of applications, such as sonar, radar, satellite communication and mobile radio, high-resolution radio channel measurements, which also supply directional information, are desirable. A mobile radio channel represents the connection between a base station and mobile stations, and deep knowledge of the channel characteristics is required in order to allow propagation and channel models to be developed and used. Such models are required by system providers to plan their networks, and the propagation environment is an essential basis for designing mobile radio systems.
Increasing numbers of subscribers and a limited number of available frequencies necessitate improved spectral efficiency. A significant improvement is obtained by using intelligent antenna arrays, such as those described in German patent application DE 195 11 751 A, for example. There, use is made of the spatial diversity inherent in the radio channel. The design and provision of radio systems with intelligent antennas necessitate high-resolution measurements of the direction information on radio channels.
Two methods, in particular, for channel investigation have become known in order to solve the problem, and these methods estimate both the delay and the azimuth of the dominant wave fronts, that is to say the most powerful wave fronts for example, which arrive at an antenna configuration. Both methods furthermore use a test signal which consists of a pulse sequence modulated by means of a pseudo-random sequence.
The following two articles are of interest: U. Martin, xe2x80x9cModeling the mobile radio channel by echo estimation,xe2x80x9d Frequenz, vol. 48, pp. 198-212, 1994; and U. Martin, xe2x80x9cEcho estimationxe2x80x94Deriving simulation models for the mobile radio channel,xe2x80x9d in Proc. IEEE Vehicular Techn. Conf., vol. 1, pp. 231-35, Chicago Ill., July 1995. They describe how the parameters of certain statistical channel models can be obtained from the results of propagation measurements. The author describes a measurement configuration in which estimates of the path delay times are made with high resolution in the frequency domain by estimation of superimposed exponential oscillations.
Alternatively, it is possible to use a method which has generally become known by the name ESPRIT, such as the 1D unitary ESPRIT method, which is disclosed in German patent application DE 195 11 752 A. If the receiving antenna of that channel measurement configuration is replaced by a centrally symmetrical antenna array, a two-dimensional (2D) unitary ESPRIT method can automatically provide estimates of both the incidence angle and the delay time for dominant signal paths. Such high-resolution direction measurements of radio channels make it easier to develop realistic channel models which include the dominant incidence directions at the base station. The 2D unitary ESPRIT method, in conjunction with this channel measurement configuration and uniform linear antenna array, has been proven in a number of field measurements, and it automatically supplies pairs of estimates of the incidence angle and of the delay time for the dominant paths, as described in U. Martin, xe2x80x9cCharakterisierung und Simulation des richtungsabhxc3xa4ngigen Funkkanalsxe2x80x9d [Characterization And Simulation Of The Directional Radio Channel], ITG Workshop on Smart Antennas, Zurich, October 1996.
The second method for channel investigation, described Fleury, Dahlhaus, Heddergott, and Tschudin, in xe2x80x9cWideband Angle Of Arrival Estimation Using The SAGE Algorithmxe2x80x9d in Proc. IEEE ISSSTA, vol. 1, pp. 79-85, Mainz, September 1996, is based on the SAGE (space-alternating generalized expectation maximization) algorithm. This iterative method provides an estimate of the parameters based on the highest probability. This method involves considerably more computation complexity than the 2D unitary ESPRIT method mentioned above, since it is based on various 1D optimization processes and requires an additional algorithm, for example that from the 2D unitary ESPRIT method to solve its initial value problem. The channel investigation method based on the 2D unitary ESPRIT algorithm (and which is also required in order to understand the invention) will therefore be explained in more detail in the following text further below.
In addition, a method which also allows the incidence direction of received wave fronts to be estimated is known from Roy and Kailath, xe2x80x9cESPRITxe2x80x94Estimation of Signal Parameters Via Rotational Invariance Techniquesxe2x80x9d in IEEE Transactions on Acoustics, Speech and Signal Processing, vol. 37, No. 7, July 1989, pages 984-995.
Finally, Josef Fuhl, et al., xe2x80x9cHigh-Resolution 3-D Direction-of-Arrival Determination for Urban Mobile Radio,xe2x80x9d IEEE Transactions on Antennas and Propagation, vol. 45, No. 4, April 1997, pages 672-682 describes a method for estimating the direction of electromagnetic waves arriving at a receiver, with the azimuth and elevation angles being determined at the same time once the propagation time delays of the electromagnetic waves have previously been determined.
The object of the invention is to provide a method and a device for measuring the characteristics of radio channels which overcome the above-noted deficiencies and disadvantages of the prior art devices and methods of this kind, and which method supplies pairs of values for the azimuth propagation time delay of the incident wave fronts with higher accuracy and less computation complexityxe2x80x94and thus more quickly as well.
With the above and other objects in view there is provided, in accordance with the invention, a method of measuring characteristics of radio channels, which comprises:
transmitting a transmission signal containing a preselected test sequence;
receiving signals with a plurality of receiving sensors in a linear antenna array, wherein respective received signals are composed of wave elements of the transmission signal with a different incidence direction and different delay;
demodulating the received signals and sampling to obtain samples;
supplying the samples for calculation of eigen vectors corresponding to dominant eigenvalues, and deriving a signal subspace matrix from the calculated eigen vectors;
producing invariance equations dependent on the signal subspace matrix; and
simultaneously determining estimated values for an incidence direction and delays of dominant wave fronts by solving the invariance equations.
Compared with the prior art methods, the invention allows the accuracy to be considerably increased and the computation complexity to be reduced. The invention can be used particularly expediently in the mobile radio field, but is not limited to this. Its advantages are also applicable, for example, to sonar applications and in radar technology.
When the azimuth and delay are being estimated, a data matrix contains a spatial invariance and a time invariance superimposed for each wave front.
The spatial frequency can be converted very easily to the incidence direction of the wave fronts at the measurement station (azimuth), and the associated time frequency can be converted very easily into the associated delay at the measurement configuration.
The invention determines the spatial and time invariances superimposed in the data matrix for each dominant wave front and converts them into corresponding angles and delays for each dominant wave front. Furthermore, the complex amplitudes can be estimated on the basis of the estimated three-dimensional parameters.
Improved results can be widely obtained if the test sequence has a chip signal form and simultaneous estimation is carried out taking account of the chip signal form of the test sequence.
Advantageous signal processing is obtained if the signals of each antenna sensor are demodulated, and sampling is then carried out with a total of Mc samples per chip in the test sequence.
In order to achieve simple signal processing, it is also expedient if the resultant samples are transformed to the frequency domain by discrete Fourier transformation, and/or the resultant values are corrected taking account of the spectrum of the chip signal form, in which case the line vectors formed from the values obtained for each sensor can be combined to form a data matrix XFxe2x80x2(n), which contains invariances which characterize the channel.
In accordance with an expedient variant of the invention, a modulated pseudo-random noise sequence is used as the transmitted signal. The use of such a pseudo-random noise sequence results in the advantage that, on the one hand, it is easy to produce and, on the other hand, is easy to evaluate.
The azimuth incidence direction is preferably measured since this has the greatest importance, particularly for mobile radio.
Simple signal processing is also achieved if the signals received by the receiving sensors are mixed to baseband before being demodulated.
In accordance with an additional feature of the invention, the sample signals are filtered, matched to the signal, since this results in the signal-to-noise ratio being optimized.
In addition, it is expedient if the received signals are oversampled, that is to say they are sampled at a sampling frequency which is greater than twice the received signal bandwidth, since this also allows an improvement in the signal-to-noise ratio to be achieved.
In order to limit the processing complexity to a reasonable level, it is worthwhile, once the samples have been transformed to the frequency domain, rejecting values below a power limit which can be predetermined.
In order to improve the time invariance structure, it is also advantageous, once they have been transformed to the frequency domain, for the samples to be corrected by division by the square of the spectrum of the chip signal form.
It may also be expedient to subject samples to smoothing in the space/frequency domain. This results in a further reduction in the computation complexity, and decorrelation of the wave fronts.
One worthwhile variant is distinguished by the use of a two-dimensional, high-resolution frequency estimation algorithm in the space/frequency domain to determine and associate the superimposed spatial and time invariances. Improved estimation accuracy is obtained by joint estimation of the azimuth and delay time of the dominant wave fronts.
In order to ensure that the estimation results are realistic, it is advisable for the two-dimensional, high-resolution frequency estimation algorithm to take account of the colored noise in the space/frequency domain.
Furthermore, the 2D unitary ESPRIT method can be used as the frequency estimation algorithm, since this method automatically gives paired 2D parameter estimated values and high estimation accuracy with little computation complexity.
It is also expedient if the complex amplitudes are estimated jointly in the space/frequency domain and/or in the space/time domain. Joint estimation in the space/frequency domain has the advantage that the estimate is made in the same domain as the other parameter estimate, while, in contrast, joint estimation in the space/time domain involves less computation complexity and gives more accurate estimation results.
In this case, it is worthwhile if the joint estimation is carried out using the weighted least squares method, since this corresponds to the realistic estimate with minimum variance. In this context, reference is had to D. G. Luenberger, Optimization by Vector Space Models, John Wiley and Sons, New York, N.Y., 1969 (pages 82-83).
With the above and other objects in view there is also provided a measurement configuration for measuring characteristics of radio channels, comprising: a linear antenna array;
a plurality M1 of receiving sensors;
each of said receiving sensors being followed, in a signal flow direction:
an analog/digital sampling device connected to a respective said sensor;
a matched filter connected to said sampling device; and
a device for discrete Fourier transformation connected to said filter;
and at least one signal processor for processing the method according to the above-outline method.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and measurement configuration for measuring the characteristics of radio channels, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.